There are many sources of energy that can be converted into electrical energy. Sources that have been explored for harvesting are wind, temperature differences, motion, light energy and radio frequency energy. In all these energy situations, a specific situation must be present, such as air flow, temperature gradient, movement, sunlight, or transmitted RF, before energy can be harvested. When these situations do not occur, there is no source for energy to be harvested.
Harvesting ambient thermal energy has not been widely explored. The characteristic of thermal energy having a peak at long infrared wavelengths poses a significant challenge for harvesting. Additionally, power management, storage control, and regulation electronics must be developed to operate efficiently and respond to the power output demand.